The invention relates to methods for reducing dynamic overvoltages in an alternating-current system.
One method of the prior art for reducing dynamic overvoltages in an alternating-current system is described in U.S. Pat. No. 3,968,419. In that document, a method for the transition from a bipolar to a monopolar mode of operation of a converter having an intermediate direct-current circuit having two metallic direct-current lines and two each series-connected static converters in the rectifier and inverter stations is described. According to that method, each of the two seriesconnected static converters is connected via one choke each to a grounding switch. Each static converter can also be short-circuited via its associated choke. There is no voltage regulation on the 3-phase side. When a static converter is short-circuited, spurious reactive-load oscillations are generated.
Regarding the relevant prior art, additional reference is made to printed publication IEE Proc., Vol. 127, Pt.C. No. 3, 1980, pp. 189-198, in which methods for reducing overvoltages which can occur when a 12-pulse high-voltage rectifier is blocked are specified. A static-converter transformer can be used, the magnetic core of which is rapidly driven into saturation with increasing current intensity, in which arrangement the magnetization characteristic of this core has a curve with a steep slope at low currents and with a shallow curve at high currents. Filters, particularly for the fifth harmonic of the fundamental frequency, and static capacitors are considered as second choices. A disadvantage of the methods specified above consists in the fact that the additional equipment required is elaborate and expensive.
Other protective devices for power converter installations are known, for example, from: GEC Journal of Science and Technology, Vol. 48, No. 3, 1982, pp. 135-140 and from Conference Paper No. 70 CP 140-PWR, IEEE Power System Engineering Committee of the IEEE Power Group, New York, N.Y., Jan. 25-30, 1970, pp. 1-8."
The system-commutated static converters of a high voltage direct-current transmission plant act as consumers of reactive power in the system. The magnitude of the reactivepower absorption is a function of the magnitude of the commutation reactances which mainly consist of the leakage reactances of the static-converter transformer, the firing angle and the amplitude of the instantaneous operating current. A part of the reactive power is supplied by the filters installed for reducing harmonics, that is to say by absorption circuits which have a capacitive characteristic with respect to the fundamental frequency. The remainder of the reactive power required is obtained either from the 3-phase system or from additional compensating devices, in most cases from capacitor banks.
If then a serious disturbance occurs in one of the two systems connected by the high-voltage direct-current transmission plant and forces a temporary interruption of the exchange of power via the high-voltage direct-current transmission plant, this means not only a change in the balance of effective power for the other, undisturbed system but also always an associated shedding of reactive load. Particularly if the connecting point of the high-voltage direct-current transmission plant to the undisturbed system without faults is a system node having a low short-circuit power (for example the end of a long 3-phase line), high dynamic overvoltages occur at this system node which can lead to plant components being endangered or to operational disturbances.